Process for epitaxially growing semiconductor crystals

ABSTRACT

A semiconductor with a P/N junction, adapted to be used as a detector or emitter of luminous radiation, is grown from a bath consisting of a liquefied mixture of three elements which are the constituents of two alloys or solid solutions taken from Groups II/VI and/or IV/VI of the Periodic Table. The proportions of the three elements in the mixture are so chosen that the solidus curve of the temperature/composition diagram intersects the stoichiometric line at a point lying along the boundary between the solid and the solid/liquid phase on the side of the lower concentrations of the element common to the two alloys. In a state of thermodynamic equilibrium for the liquid/solid phase, the bath is slowly cooled in a temperature range above or below a critical temperature corresponding to the point of intersection with resulting growth of an N-type or P-type layer on a substrate immersed in the bath. By a change in the composition, with or without a shift in temperature, the conductivity type of the layer is altered with formation of a P/N junction.

United States Patent [191 Moulin Feb. 27, 1973 PROCESS FOR EPITAXIALLYGROWING SEMICONDUCTOR CRYSTALS [75] Inventor: Michel Moulin,

France [73] Assignee: Thomson-CSF [22] Filed: Dec. 15, 1970 [21] Appl.No.: 98,262

Chilly-Mazarin,

[58] Field of Search..'.148/l7l, 17 2, 1.5; 252/623 T; 117/113, 160,200, 227, 201

[56] References Cited UNITED STATES PATENTS 3,403,133 9/1968 Fredrick etal ..252/62.3 T

OTHER PUBLICATIONS Hiscoclgs et al., Crystal Pulling and ggn stitutionin PbixSnxTeJournal of Materials Science, Vol. 3, 1968, pages 76-79.

Primary Examiner-Robert D. Edmonds Attorney-Karl F. Ross [5 7 ABSTRACT Asemiconductor with a PIN junction, adapted to be used as a detector oremitter of luminous radiation, is grown from a bath consisting of aliquefied mixture of three elements which are the constituents of twoalloys or solid solutions taken from Groups lI/VI and/or IV/Vl of thePeriodic Table. The proportions of the three elements in the mixture areso chosen that the solidus curve of the temperature/composition diagramintersects the stoichiometric line at a point lying along the boundarybetween the solid and the solid/liquid phase on the side of the lowerconcentrations of the element common to the two alloys. In a state ofthermodynamic equilibrium for the liquid/solid phase, the bath is slowlycooled in a temperature range above or below a critical temperaturecorresponding to the point of intersection with resulting growth of anN- type or P-type layer on a substrate immersed in the bath. By a changein the composition, with or without a shift in temperature, theconductivity type of the layer is altered with formation of a PINjunction.

9 Claims, 7 Drawing Figures PATENIEUFEBZYIW 3,718,511

SHEET 15F 2 FIG. I

FIG. 2

FIG. 3

Michel Moulin Attorney PROCESS FOR EPITAXIALLY GROWING SEMICONDUCTORCRYSTALS My present invention relates to a process for epitaxiallygrowing semiconductor crystals of predetermined conductivity type, morespecifically a unitary crystal body with at least two zones of oppositeconductivity types forming a P/N junction therebetween.

Conventional processes for producing such semiconductors, by introducingimpurities or altering the lattice structure of the crystalline bodythrough alloying or diffusion, do not yield crystals of exactlypredictable and reproducible quality. This quality depends on theelectronic properties and other parameters of the starting material,such as the width of the forbidden band, which cannot always beaccurately predetermined and which require a corrective heat treatment;however, the subsequent handling of the material at elevatedtemperatures tends to alter the characteristics of the product in anunfavorable manner, as by objectionably increasing the number of chargecarriers in the n-type of p-type zone.

The general object of my invention is to provide an improved process formaking such semiconductors with avoidance of the aforestated drawbacks.

A more specific object is to provide a process for reproduciblymanufacturing a crystal structure adapted to be used as a detector oremitter of luminous radiation.

In accordance with my present invention, I utilize the phenomenon ofexpitaxial crystal growth from a suitable solution on a compatiblesubstrate immersed therein, with control of the bath composition and theoperating temperatures, to obtain a crystalline layer of the desiredconductivity type which may be the same as or different from that of thesubstrate and which may be followed by the formation of a second crystallayer, of opposite conductivity type, upon a modification of the bathcomposition and without removal of the treated substrate therefrom.Thus, the resulting crystal body may have a junction between the portionthereof constituted by the original substrate and a layer of oppositeconductivity type grown thereon, and/or between two such layers grownsuccessively in the same bath. It is also possible to replace orsupplement such a PIN junction by a so-called homojunction formedbetween zones of like conductivity type but different concentration ofcharge carriers to produce a resistance differential, the transitionfrom one zone to the next being again accomplished by a suitablemodification of the bath composition.

More particularly, the process according to my invention starts with theselection of two semiconductive compositions of two constituents each,these constituents being normally solid elements taken from Groups Il/VIand/or IV/Vl of the Periodic Table and including one element common toboth compositions. It is preferred to utilize either selenium ortellurium, both from Group VI, as one of the elements and to choose theother two elements of the composition from among such metals as lead andtin (Group IV) and/or cadmium, zinc or mercury (Group II).

An important consideration for the choice of these elements is therequirement that they form alloys or solid solutions which can berepresented by a tempera ture/composition diagram whose stoichiometricline is intersected by the solidus curve in certain compositionsdescribed hereinafter, the point of intersection lies along the boundarybetween the solid and the solid/liquid phases on the side of the lowerconcentrations of the common element, the liquidus curve of the diagramdiverging from that boundary in the direction of these lowerconcentrations so as to be substantially spaced therefrom at thetemperature level of this point of intersection.

Next, a bath consisting of a liquefied mixture of these threeconstituents is prepared in proportions corresponding to a point on theliquidus curve which is spaced from a neutral point on that curve, i.e.,the one lying on the temperature level of the aforementioned point ofintersection, in a direction consistent with the desired conductivitytype of the crystal layer to be grown; this is the point of saturationand incipient solidification occurring upon a suitable lowering of thebath temperature. Now, a substrate of compatible crystal structure (suchas a conventionally produced semiconductor body of the same basiccomposition) is immersed in the bath which is thereupon cooled at acontrolled rate resulting in the growing of the desired layer on thesubstrate. Upon the attainment of a final temperature, still remote fromthe level of the neutral point on the liquidus curve, the controlledcooling is terminated at least temporarily, with or without immediateremoval of the coated substrate from the bath according to the number oflayers to be formed.

If a second layer is required, the proportion of the bath constituentsis modified before further cooling. Thus, if a PIN junction is to beformed between the two layers, the modification may be such as to shiftthe saturation point on the liquidus curve to a location on the oppositeside of the neutral point, generally in the direction of decreasingtemperatures with resulting reliquefaction of the bath mixture at theaforementioned final temperature; alternatively, the modification mayvary the position of the solidus curve so as to displace its point ofintersection with the stoichiometric line to a location a the oppositeside of the final temperature level previously reached, thus again witha reversal of the relative positions of the neutral point and thesaturation point on the liquidus curve. In the first instance, theproportion of the common element with reference to the combinedproportion of the two other elements may be reduced, preferably by theintroduction of added amounts of these latter two elements withsubstantially no change of their relative proportion in the bath; in thesecond instance, the relative proportion of the last-mentioned elementsmay be varied with substantially no change in the proportion of thecommon element with reference to these other two. Both measures could,however, also be used jointly.

The invention will be described in greater detail hereinafter withreference to the accompanying drawing in which:

FIGS. 1 and 2 are temperature/composition diagrams of differenttwo-component mixtures to be used as starting materials for a processaccording to the invention;

FIGS. 3 and 4 are similar diagrams for a three-component compositionconsisting of the constituents of the mixtures of FIGS. 1 and 2; and

FIGS. 5 '7 are somewhat schematic cross-sectional views of semiconductorbodies obtained by the process according to my invention.

In FIG. 1 I have shown the phase diagram of a leadtellurium alloy, withthe proportion of lead in terms of atomic concentration decreasing from100 percent to percent from left to right and with the correspondingproportion of tellurium similarly decreasing from right to left. Thediagram shows, at the 50 percent value, a stoichiometric line l(denoting the intrinsic semiconductor material corresponding to thiscomposition) along with a solidus curve 11 and a liquidus curve Illseparating a liquid phase (liq), a liquid-solidphase (ligsol) and asolid phase (sol). To the left of line l there is shown a region Nrepresenting n-type conductivity for compositions with a relatively lowtellurium content, the opposite region P on the right denoting p-typeconductivity with a high proportion of tellurium. It will be noted thatsolidus curve II does not intersect the stoichiometric line I at anytemperature plotted on the diagram, except at the common vertex of thetwo curves.

FIG. 2 shows a corresponding diagram for a mixture of tin and tellurium.Here the solidus curve 11 is offset to the right from stoichiometricline I, i.e., in the direction of increasing percentages oftellurium(region P), without ever"intersecting that line.

The diagram of FIG. 3 relates to a mixture of the two compositionsrepresented in FIGS. 1 and 2, i.e., a three-component alloy consistingof lead, tin and tellurium. The combined proportion of lead and tindecreases from 100 to 0 percent from left to right, the proportion oftellurium (the common element of the two starting compositions)decreasing again from right to left in this diagram. We may consider thecomposition of the mixture as given by with the parenthetical termsexpressing the respective atomic concentrations.

The solidus curve 11 of FIG. 3 intersects the stoichiometric line I at apoint lying on atemperature level T, (about 470 C); this point ofintersection 10 is located on the boundary between the liquid-solid andthe solid phase at the left of the diagram, thus on the side of thelower percentages of the common component Te. On the same temperaturelevel T, there exists on the liquidus curve 111 a neutral point 20corresponding to a relatively low proportion u, of (Te) and (Pb Sn). Therelative proportion x of lead and tin in that mixture does not affectthe location of points 10 and 20 so long as the overall proportion u ismaintained constant.

If the bath composition were such as to correspond to the ratio u,, acooling of the liquid from a more elevated temperature to the level T,would lead to incipient solidification at the neutral point 20, with agrowth ofa layer of intrinsic (high-resistance) material on an immersedsubstrate. In accordance with this invention, however, l choose aninitial composition corresponding to either a ratio such as u,,, with asaturation point 21 on a temperature level T, (in a range of 500 to 550C) corresponding to a point 11 on solidus curve 11 well above point 10and within the n-type region N of the diagram, or a ratio such as u witha saturation point 22 on a temperature level T, (in a range of 400 to450 C) corresponding to a point 12 on curve 11 well below point 10 andwithin the p-type region P.

As illustrated in FIG. 4, a change in the relative proportion of leadand tin with maintenance of the stoichiometrically effective overallratio 11, results in a shifting of the solidus curve ll either towardthe left, thus into a position II' closer to that of the Pb/Te diagramof FIG. I, or toward the right, i.e., into a position ll closer to thatof the Sn/Te diagram of FIG. 2. Curve II intersects the line I at apoint 10' and the temperature level T, at a point 11' within the n-typeregion N; curve Il" intersects the line I at a point 10" and thetemperature level Te, at a point 12" within the p-type region P.

Thus, modifying x instead of u also shifts the relative position of thesaturation point and the neutral point on the liquidus curve. With adisplacement of the point of intersection 10 to either the position 10or the position 10" in FIG. 4, the neutral point 20 is similarlydisplaced to a position 20' or 20" so that either n-type or p-typedeposits can be obtained with an initial bath composition u, and with atemperature in the neighborhood of level T,.

The following Examples serve to illustrate the several aspects of myinvention:

EXAMPLE I It is desired to form a junction between a p-typemonocrystalline substrate of composition (Pb Sn, )1 and.

The substrate is a wafer out along a privileged crystal plane from asuitably doped p-type body of the composition stated, produced by aconventional crystaldrawing process.

The bath chosen or the formation of the junction has the composition (PbSn Se corresponding to x 0.1 land u 0.05. This mixture is heated in aprotective atmosphere of argon to a temperature of 800 C, above theliquidus curve on the PbSn side of the associated phase diagram which isgenerally similar to that of FIGS. 3 and 4.

This bath is cooled to a level of about 700 C corresponding to the point21 in the diagram of FIG. 3. At

this point, the substrate is immersed into the saturated solution fromwhich a layer consisting predominantly of lead and tin begins tocrystallize on the substrate. Next, the bath temperature isprogressively lowered over a period of about 10 minutes to about 650 C,well above the level T,. The substrate is then removed and is found tohave an epitaxial layer of the same basic composition (Pb,S, Se) as thesubstrate, with a lead/tin ratio corresponding to X=0.06, which is ofn-type conductivity. The growth rate of this layer is on the order of 2microns per minute.

EXAMPLE II It is desired to produce a semiconductor of the same generaltype as that obtained in Example I, with two epitaxial layers ofp and ntype, respectively, separated by a P/N junction. The first, n-type layeris produced in the same manner as in the preceding Example. When thefinal bath temperature of 650 C is reached, however, the substrate isnot removed but the composition of the bath is modified by theintroduction of a sufficient quantity of a lead/tin alloy to change thepropor tion of selenium from a value u 0.05 (corresponding to u,, inFIG. 3) to a value u =0.03 (corresponding to u, in FIG. 3), withoutchange in the magnitude of x. With these altered proportions, theoperating point is shifted to the left of the liquidus curve III so thatthe mixture is reliquefied, requiring further cooling to about 600 Crestore saturation at a new point of incipient solidificationcorresponding to point 22 of FIG. 3.

Thereafter, controlled cooling is resumed for a period of, say, 20minutes with formation of a second, p-type layer (with x 0.075) on then-type layer already present on the substrate which is then removed fromthe bath.

EXAMPLE III The semiconductor body described in the preceding Examplecan be produced by modifying the bath concentration, after formation ofthe n-type first layer in the manner. described, by introducing asufficient amount of selenium and tin to change the value of x from 0.11to 0.15, with u remaining at its original value of 0.05. Thisestablishes a new solidus curve, similar to curve II of FIG. 4, to theright of the original curve whereby the deposit obtained upon furthercontrolled cooling is of p-type conductivity as explained above.

EXAMPLE IV To produce a semiconductor akin to that of Example I but withthe selenium replaced by tellurium, a bath consisting of lead, tin andtellurium as discussed in conjunction with FIGS. 3 and 4 is used with acomposition (Pb Sn Te i.e., with x 0.30 and u 0.05. The substrate, inthis case, is a conventionally drawn monocrystal composed of lead, tinand tellerium. The n-type layer grown on that substrate is ofsubstantially the same composition, with x 0.20 and Controlled coolingtakes place from 550 to about 500 Cl Again, a second (p-type) layer maybe deposited on the n-type first layer by the technique of Example [I orIII.

In FIGS. 5 7 I have shown several types of semiconductor obtainable withthe process described above. According to FIG. 5, a substrate 30 ofp-type conductivity is covered by an epitaxially grown n-type layer 31forming therewith a junction 32, the two major faces of beingelectrically-energized in the forward direction of one of theirjunctions. To obtain a laser-type stimulation of this emission, it isdesirable to subdivide an exposed layer, as illustrated in FIG. 7 forthe layer 31, in two dimensions into a multiplicity of small segments ofapproximately cubic shape measuring, for example, 0.5 mm on each side.These cubes, which may be produced by etching through a mask of siliconoxide, are provided on all lateral faces with semireflecting coatings 37to intensify the emission of substantially monochromatic light from theplane of junction 32.

The concentration of charge carriers and the physical thickness of thevarious layers depends on the intended use of the semiconductor. Forphotovoltaic detection, for example, the light-receiving layer (31 inFIG. 5) should be only limitedly conductive, i.e., should deviate onlyslightly in its composition from the stoichiometric relationship, andshould have a thickness depending on the absorptivity of the materialfor the wavelengths to be detected, the adjoining zone of oppositeconductivity type being given a strong concentration of charge carriersto minimize current flow in the nonilluminated state. The use ofhomojunction, as described above, in the position of boundary 36 (thuswith additional n-type material of different carrier concentration onthe segments of FIG. 7) may help solve the problem of guiding theradiation in an emitter of light, particularly if this emitter has anactive layer 31 insufficiently transparent to this radiation.

The process described hereinabove does not exclude, of course, thepossibility of additionally doping one or more of the layers byconventional techniques to modify their carrier concentrations.

For the detection and emission of infrared radiation, particularlysuitable compositions include (Cd, Hg)Te in addition to thelead/tin/tellurium and lead/tin/selenium alloys discussed above. For thevisible spectrum, Sn(Se,Te) is preferred.

I claim:

l. A process for epitaxially growing a semiconductor crystal ofpredetermined conductivity type, comprising the steps of:

selecting two semiconductive compositions of two normally solid elementseach, including one element common to both compositions, withconstituents from Groups II/VI or IV/VI of the Periodic Table, saidconstituents being cadmium or mercury in Group II, lead or tin in GroupIV and selenium or tellurium in Group VI, said common element beingselenium, tellurium or zinc, the other two elements being members of thesame Group;

preparing a bath consisting of a liquefied mixture of the constituentsof said compositions in proportions giving rise to atemperature/composition diagram with a stoichiometric line and with asolidus curve intersecting said line at a first point lying along theboundary between the solid and the solid/liquid phases on one side ofthe solidus curve, said diagram having a liquidus curve diverging fromsaid boundary and defining a second point on the temperature level ofsaid first point, said proportions being chosen to correspond to a thirdpoint on said liquidus curve spaced from said second point in adirection consistent with said predetermined conductivity type;

lowering the temperature of said bath from an elevated level to a levelof incipient solidification corresponding to said third point;

immersing into said bath a substrate of a crystal structure compatiblewith that of a solid mixture of said constituents;

progressively cooling said bath at a controlled rate with growth of alayer of said predetermined conductivity type on said substrate;

and terminating the controlled cooling of said bath at a finaltemperature remote from the level of said first and second points withsubsequent removal of said substrate therefrom.

2. A process as defined in claim 1 wherein, following termination ofcontrolled cooling and prior to removal of said substrate, theproportion of said constituents is modified in said bath to reverse saidconductivity type, with subsequent continuation of controlled coolingand formation of another layer of opposite conductivity type on saidsubstrate.

3. A process as defined in claim 2 wherein the modification of the bathcomposition involves a diminution of the proportion of said commonelement with reference to the combined proportion of the other twoelements, with substantially no change in the relative proportion ofsaid other two elements and with resulting reliquefaction of the mixtureat said final temperature above a fourth point on said liquidus curvewhich is spaced from said second point in a direction opposite saidthird point and consistent with said opposite conductivity type, thebath temperature being lowered to said fourth point prior to resumptionof controlled cooling to form said other layer.

4. A process as defined in claim 2 wherein the modification of the bathcomposition involves the substantial maintenance of the originalproportion of said common element with reference to the combinedproportion of the other two elements, with a change in the relativeproportion of said other two elements resulting in a shifting of saidboundary and corresponding displacement of said first point sufficientto move said second point onto the opposite side of said third point,controlled cooling being resumed without reliquefaction of the mixture.

5. A process as defined in claim 1 wherein said common element istellurium or selenium and the other two elements are lead and tin.

6. A process as defined in claim 5 wherein the initial bath compositionis substantially (Pb Sn Te 7. A process as defined in claim 5 whereinthe initial bath composition is substantially (Pb Sn Se 8. A process asdefined in claim 1 wherein said common element is tellurium and theother two elements are cadmium and mercury.

9. A process as defined in claim 1 wherein said common element is zincand the other two elements are selenium and tellurium.

2. A process as defined in claim 1 wherein, following termination ofcontrolled cooling and prior to removal of said substrate, theproportion of said constituents is modified in said bath to reverse saidconductivity type, with subsequent continuation of controlled coolingand formation of another layer of opposite conductivity type on saidsubstrate.
 3. A process as defined in claim 2 wherein the modificationof the bath composition involves a diminution of the proportion of saidcommon element with reference to the combined proportion of the othertwo elements, with substantially no change in the relative proportion ofsaid other two elements and with resulting reliquefaction of the mixtureat said final temperature above a fourth point on said liquidus curvewhich is spaced from said second point in a direction opposite saidthird point and consistent with said opposite conductivity type, thebath temperature being lowered to said fourth point prior to resumptionof controlled cooling to form said other layer.
 4. A process as definedin claim 2 wherein the modification of the bath composition involves thesubstantial maintenance of the original proportion of said commonelement with reference to the combined proportion of the other twoelements, with a change in the relative proportion of said other twoelements resulting in a shifting of said boundary and correspondingdisplacement of said first point sufficient to move said second pointonto the opposite side of said third point, controlled cooling beingresumed without reliquefaction of the mixture.
 5. A process as definedin claim 1 wherein said common element is tellurium or selenium aNd theother two elements are lead and tin.
 6. A process as defined in claim 5wherein the initial bath composition is substantially(Pb0.70Sn0.30)0.95Te0.05.
 7. A process as defined in claim 5 wherein theinitial bath composition is substantially (Pb0.89Sn0.11)0.95Se0.05.
 8. Aprocess as defined in claim 1 wherein said common element is telluriumand the other two elements are cadmium and mercury.
 9. A process asdefined in claim 1 wherein said common element is zinc and the other twoelements are selenium and tellurium.